Display waveguide with a high-index portion

ABSTRACT

A display waveguide configured for conveying image light to a viewer has a waveguide body which refractive index varies in the thickness direction to include a high-index region between lower-index regions. Multi-layer and gradient index implementations are described. The waveguide transmits a portion of image light within the high-index region of the waveguide to provide a wider field of view.

REFERENCE TO RELATED APPLICATION

This application claims priority to US Provisional Application No.62/926,053 entitled “Display Waveguide with a High-Index Layer” filed onOct. 25, 2019 and incorporated herein by reference in its entirety.

TECHNICAL FIELD

The present disclosure generally relates to optical display systems anddevices, and in particular to waveguide displays and componentstherefor.

BACKGROUND

Head mounted displays (HMD), helmet mounted displays, near-eye displays(NED), and the like are being used increasingly for displaying virtualreality (VR) content, augmented reality (AR) content, mixed reality (MR)content, etc. Such displays are finding applications in diverse fieldsincluding entertainment, education, training and biomedical science, toname just a few examples. The displayed VR/AR/MR content can bethree-dimensional (3D) to enhance the experience and to match virtualobjects to real objects observed by the user. Eye position and gazedirection, and/or orientation of the user may be tracked in real time,and the displayed imagery may be dynamically adjusted depending on theuser's head orientation and gaze direction, to provide a betterexperience of immersion into a simulated or augmented environment.

Compact display devices are desired for head-mounted displays. Because adisplay of HMD or NED is usually worn on the head of a user, a large,bulky, unbalanced, and/or heavy display device would be cumbersome andmay be uncomfortable for the user to wear.

Projector-based displays provide images in angular domain, which can beobserved by a user's eye directly, without an intermediate screen or adisplay panel. An imaging waveguide may be used to carry the image inangular domain to the user's eye. The lack of a screen or a displaypanel in a projector display enables size and weight reduction of thedisplay.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments disclosed herein will be described in greater detail withreference to the accompanying drawings which represent exampleembodiments thereof, in which like elements are indicated with likereference numerals, and wherein:

FIG. 1 is a schematic isometric view of a waveguide display system usinga waveguide assembly for transmitting images to a user;

FIG. 2 is a schematic diagram illustrating the coupling of a colorchannel into a display waveguide and an input FOV therefor;

FIG. 3 is a schematic diagram illustrating input and output FOVs of adisplay waveguide for a selected color channel;

FIG. 4 is a schematic side cross-section of a display waveguide with twoout-coupler gratings at main outer faces;

FIG. 5 is a schematic plan view of a pupil-expanding waveguideillustrating an example layout of output-coupler gratings and anin-coupler aligned therewith;

FIG. 6A is a schematic k-space diagram illustrating the formation of a2D FOV in an example embodiment of the waveguide of FIG. 5;

FIG. 6B is a graph illustrating the 2D FOV of the waveguide of FIG. 6Ain the angle space;

FIG. 7 is a normalized K-space diagram illustrating an estimation of asymmetric 1D FOV shared by two different color channels for a displaywaveguide of FIG. 5;

FIG. 8 is a schematic side cross-sectional view of a display waveguideillustrating the divergence of two different color channels therein;

FIG. 9A is a schematic side cross-sectional view of a display waveguidehaving a top high-index layer illustrating the coupling of differentcolor channels in to the waveguide;

FIG. 9B is a schematic side cross-sectional view of the displaywaveguide of FIG. 9A illustrating example propagation trajectories inthe waveguide of in-coupled rays of a selected wavelength in dependenceon the angle of incidence upon the waveguide;

FIG. 10 is a K-space diagram illustrating the coupling of a wavelengthof image light into the two-layer display waveguide of FIGS. 9A and 9B;

FIG. 11 is a graph illustrating critical angles of incidence forin-coupling of red and blue wavelengths into the two-layer waveguide ofFIGS. 9A and 9B in dependence on the pitch of the input grating;

FIG. 12 is a schematic side cross-sectional view of a two-layer displaywaveguide with a capping layer over the interface grating;

FIG. 13 is a schematic side cross-sectional view of a three-layerdisplay waveguide illustrating the coupling of rays of image light atopposite ends of the FOV;

FIG. 14 is a schematic side cross-sectional view of a multi-layerdisplay waveguide with five layers of different refractive indices;

FIG. 15 is a schematic side cross-sectional view of a gradient-indexdisplay waveguide;

FIG. 16 is a schematic side cross-sectional view of a two-waveguidestack including a two-layer waveguide;

FIG. 17A is a schematic plan view of a binocular NED with twopupil-expanding waveguides and in-couplers diagonally offset from exitpupils of the out-couplers;

FIG. 17B is a schematic vector diagram illustrating grating vectors forthe example layout of FIG. 17A;

FIG. 18A is an isometric view of a head-mounted display of the presentdisclosure;

FIG. 18B is a block diagram of a virtual reality system including theheadset of FIG. 18A.

DETAILED DESCRIPTION

In the following description, for purposes of explanation and notlimitation, specific details are set forth, such as particular opticaland electronic circuits, optical and electronic components, techniques,etc. in order to provide a thorough understanding of the presentinvention. However, it will be apparent to one skilled in the art thatthe present invention may be practiced in other embodiments that departfrom these specific details. In other instances, detailed descriptionsof well-known methods, devices, and circuits are omitted so as not toobscure the description of the example embodiments. All statementsherein reciting principles, aspects, and embodiments, as well asspecific examples thereof, are intended to encompass both structural andfunctional equivalents thereof. Additionally, it is intended that suchequivalents include both currently known equivalents as well asequivalents developed in the future, i.e., any elements developed thatperform the same function, regardless of structure.

Note that as used herein, the terms “first”, “second”, and so forth arenot intended to imply sequential ordering, but rather are intended todistinguish one element from another, unless explicitly stated.Similarly, sequential ordering of method or process steps does not implya sequential order of their execution, unless explicitly stated.

Furthermore, the following abbreviations and acronyms may be used in thepresent document: HMD (Head Mounted Display); NED (Near Eye Display); VR(Virtual Reality); AR (Augmented Reality); MR (Mixed Reality); LED(Light Emitting Diode); FOV (Field of View); TIR (Total InternalReflection); HI (High Index). The terms “NED” and “HMD” may be usedherein interchangeably.

Example embodiments may be described hereinbelow with reference topolychromatic light that is comprised of three distinct color channels.The color channel with the shortest wavelengths may be referred to asthe blue (B) channel or color, and may represent the blue channel of anRGB color scheme. The color channel with the longest wavelengths may bereferred to as the red (R) channel or color and may represent the redchannel of the RGB color scheme. The color channel with wavelengthsbetween the red and blue color channels may be referred to as the green(G) channel or color, and may represent the green channel of the RBGcolor scheme. The blue light or color channel may correspond towavelength about 500 nm or shorter, the red light or color channel maycorrespond to wavelength about 625 nm or longer, and the green light orcolor channel may correspond to a wavelength range 500 nm to 565 nm. Itwill be appreciated however that the embodiments described herein may beadapted for use with polychromatic light comprised of any combination oftwo or more, or preferably three or more color channels, which mayrepresent different portions of a relevant optical spectrum.

An aspect of the present disclosure relates to a display systemcomprising a waveguide and an image light source coupled thereto,wherein the waveguide is configured to receive image light emitted bythe image light source and to convey the image light received in a fieldof view (FOV) of the waveguide to an eyebox for presenting to a user.The term “field of view” (FOV), when used in relation to a displaysystem, may relate to an angular range of light propagation supported bythe system or visible to the user. A two-dimensional (2D) FOV may bedefined by angular ranges in two orthogonal planes. For example, a 2DFOV of a NED device may be defined by two one-dimensional (1D) FOVs,which may be a vertical FOV, for example +\−20° relative to a horizontalplane, and a horizontal FOV, for example +\−30° relative to the verticalplane. With respect to a FOV of a NED, the “vertical” and “horizontal”planes or directions may be defined relative to the head of a standingperson wearing the NED. Otherwise the terms “vertical” and “horizontal”may be used in the present disclosure with reference to two orthogonalplanes of an optical system or device being described, without implyingany particular relationship to the environment in which the opticalsystem or device is used, or any particular orientation thereof to theenvironment.

An aspect of the present disclosure relates to a display waveguide thatis configured for conveying image light to a viewer and that has awaveguide body which refractive index varies in the thickness directionto include a high-index region between lower-index regions. Multi-layerand gradient index implementations are described. The waveguidetransmits a portion of image light within the high-index region of thewaveguide to provide a wider field of view.

An aspect of the present disclosure relates to a waveguide for conveyingimage light in a display system, the waveguide comprising a waveguidebody having two outer surfaces and a thickness therebetween, thewaveguide body comprising an input area and an output area, thewaveguide body configured to guide the image light received at the inputarea toward the output area, wherein the waveguide body has a refractiveindex that varies in a direction of the thickness and is greater in amiddle portion of the waveguide body between the outer surfaces than inportions of the waveguide body adjacent to the outer surfaces. Thewaveguide body may be contiguous. An input coupler may be disposed inthe input area and configured to couple the image light into thewaveguide body for propagating toward the output area. An output couplermay be disposed in the output area and configured to couple the imagelight out of the waveguide body for propagating toward an eyebox.

In some implementations the input coupler may be configured to couplethe image light into the waveguide body in a range of propagation anglescomprising a first propagation angle and a second propagation angle, sothat first rays of the image light coupled into the waveguide body atthe first propagation angle are guided toward the output area by totalinternal refraction at the outer surfaces of the waveguide body, whilesecond rays of the image light coupled at the second propagation angleare guided within the middle portion of the waveguide body.

In some implementations the input coupler may comprise a first inputgrating disposed at least in part in the middle portion of the waveguidebody so as to inject the image light directly into the middle portion.The output coupler may comprise a first output grating disposed at leastin part in the middle portion of the waveguide body. In someimplementations the output coupler may comprise a second output gratingconfigured to redirect the image light received from the first outputgrating out of the waveguide body. The second output grating may bedisposed at least in part in the middle portion of the waveguide body.

In some implementations the middle portion of the waveguide body maycomprise a middle layer, and the waveguiding body may further comprisetwo outer layers sandwiching the middle layer, wherein the middle layerhas a greater refractive index than the two outer layers. In someimplementations at least one of the first and second output gratings maybe disposed in the middle layer or at a surface thereof. In someimplementations the waveguide body may comprise a plurality of layers,the plurality of layers comprising at least three layers havingdifferent refractive indices.

In some implementations the refractive index of the waveguide bodyvaries from at least 2.3 in the middle portion to at most 1.9 at theouter surfaces thereof. In some implementations the refractive index ofthe waveguide body gradually varies in the thickness dimension,decreasing from a maximum value in the middle portion of the waveguidebody toward lesser values at the two outer surfaces thereof.

An aspect of the disclosure relates to a display device comprising: asupport structure for wearing on a head of a user; a light projectorcarried by the support structure and configured to emit image light;and, a first waveguide carried by the support structure and configuredto convey at least a first portion of the image light from the lightprojector to an eyebox. The first waveguide may comprise a waveguidebody having two outer surfaces and a thickness therebetween, thewaveguide body comprising an input area and an output area, thewaveguide body configured to guide the image light received at the inputarea toward the output area, wherein the waveguide body has a refractiveindex that varies in a direction of the thickness and may be greater ina middle portion of the waveguide body between the outer surfaces thanin portions of the waveguide body adjacent to the outer surfaces. Thefirst waveguide may further comprise an input coupler configured tocouple the first portion of the image light into the waveguide body, andan output coupler configured to couple the first portion of the imagelight out of the waveguide body for propagating toward an eyebox.

In some implementations of the display device, the input coupler maycomprise a first input grating disposed at least in part in the middleportion of the waveguide body, and the output coupler may comprise afirst output grating disposed at least in part in the middle portion ofthe waveguide body, wherein the first output grating may be spaced apartfrom the first input grating in a first direction that may be orthogonalto a direction of the thickness.

In some implementations of the display device wherein the image lightcomprises a plurality of color channels, the first waveguide may beconfigured to couple at least a first color channel of the image lightfor guiding toward the eyebox.

In some implementations the display device may comprise a secondwaveguide carried by the support structure and configured to convey atleast a second portion of the image light from the light projector tothe eyebox, wherein the image light comprises a first color channel anda second color channel, and wherein the first waveguide may beconfigured to transmit the first color channel and the second waveguidemay be configured to transmit the second color channel.

An aspect of the disclosure relates to a method for conveying imagelight from an image light source to an eyebox, the method comprising:directing the image light onto an input area of a waveguide comprisingtwo opposed outer surfaces and a middle portion therebetween, the middleportion extending from the input area of the waveguide to an output areathereof, the middle portion having a greater refractive index thanportions of the waveguide body adjacent to the opposed outer surfacesthereof; coupling the image light into the waveguide body at a range ofpropagation angles comprising a first propagation angle and a secondpropagation angle; and, propagating the image light in the waveguidefrom the input area to the output area so that first rays of the imagelight coupled into the waveguide at the first propagation angle areguided toward the output area by total internal refraction at theopposed outer surfaces of the waveguide, while second rays of the imagelight coupled into the waveguide at the second propagation anglepropagate toward the output area within the middle portion of thewaveguide body. The method may comprise using at least one outputgrating disposed in the middle portion of the waveguide body to couplethe second rays out of the waveguide body. The method may compriseguiding at least one color channel of the image light predominantlywithin the middle portion of the waveguide.

An aspect of the present disclosure provides a waveguide for conveyingimage light in a display system, the waveguide comprising: a waveguidebody having two outer surfaces and a thickness therebetween, thewaveguide body comprising an input area and an output area, thewaveguide body configured to guide the image light received at the inputarea toward the output area, wherein the waveguide body has a refractiveindex that varies in a direction of the thickness. An input coupler maybe disposed in the input area and configured to couple the image lightinto the waveguide body for propagating toward the output area. Anoutput coupler may be disposed in the output area and configured tocouple the image light out of the waveguide body for propagating towarda viewing area.

In some implementations the refractive index may be greater at one ofthe outer surfaces than at the other of the outer surfaces. In someimplementations the refractive index may be greater in a middle portionof the waveguide body between the outer surfaces than in portions of thewaveguide body adjacent to the outer surfaces.

Example embodiments of the present disclosure will now be described withreference to a waveguide display. Generally a waveguide display mayinclude an image light source such as a pixilated electronic display ora scanning projector assembly, a controller, and an optical waveguideconfigured to transmit image light from the image light source to anexit pupil for presenting images to a user. The image light source mayalso be referred to herein as a display projector, an image projector,or simply as a projector. Example display systems incorporating adisplay waveguide wherein features and approaches disclosed here may beused, include, but not limited to, a near-eye display (NED), a head-updisplay (HUD), a head-down display, and the like.

With reference to FIG. 1, there is illustrated a waveguide display 100in accordance with an example embodiment. The waveguide display 100includes an image light source 110, a waveguide 120, and may furtherinclude a display controller 155.

The image light source 110, which may be referred to herein as projector110, is configured to emit image light 111. In some embodiments theimage light source 110 may be in the form of, or include, a scanningprojector. In some embodiments the scanning projector may include alight source, such as but not limited to a laser diode (LD) or alight-emitting diode (LED), and one or more scanning reflectors. In someembodiments the scanning projector may include a scanning light source.In some embodiments the image light source 110 may include a pixelatedmicro-display, such as for example but not limited to a liquid crystaldisplay (LCD), an organic light emitting display (OLED), an inorganiclight emitting display (ILED), an active-matrix organic light-emittingdiode (AMOLED) display, or a transparent organic light emitting diode(TOLED) display. In some embodiment the image light source 110 mayinclude a linear array of light sources, such LEDs, LDs, or the like. Insome embodiments it may include a 2D pixel array, and each pixel may beconfigured to emit polychromatic light. The image light source 110 mayfurther include one or more optical components configured to suitablycondition the image light. This may include, without limitation,expanding, collimating, correcting for aberrations, and/or adjusting thedirection of propagation of the image light, or any other suitableconditioning as may be desired for a particular system and electronicdisplay. The one or more optical components in the optics block mayinclude, without limitations, one or more lenses, mirrors, apertures,gratings, or a combination thereof. In some embodiments the optics blockof the image light source 110 may include one or more adjustableelements operable to scan the beam of light with respect to itspropagation angle.

The waveguide 120 may comprise a waveguide body 123, an input coupler130 in an input area of the waveguide, and an output coupler 140 in anoutput area of the waveguide. In some embodiments a waveguide stackcomposed of two or more waveguides that are stacked one over another maybe used in place of the waveguide 120. The input coupler 130 may bedisposed at a location where it can receive the image light 111 from theimage light source 110. The input coupler 130, which may also bereferred to herein as the in-coupler 130, is configured to couple theimage light 111 into the waveguide 120, where it propagates toward theoutput coupler 140. The output coupler 140, which may also be referredto herein as the out-coupler, may be offset from the input coupler 130and configured to de-couple the image light from the waveguide 120 forpropagating in a desired direction, such as for example toward a user'seye 166. The out-coupler 140 may be greater in size than the in-coupler130 to expand the image beam in size as it leaves the waveguide, and tosupport a larger exit pupil than that of the projector 110. In someembodiments the waveguide body 123 may be partially transparent tooutside light, and may be used in AR applications. The waveguide 120 maybe configured to convey a two-dimensional (2D) FOV from the inputcoupler 130 to the output coupler 140, and ultimately to the eye 166 ofthe user. Here and in the following description the display waveguide120 and embodiments thereof may be described with reference to aCartesian coordinate system (x,y,z), in which the (x,y) plane isparallel to the outer faces of the waveguide through which the waveguidereceives and/or outputs the image light, and the z-axis is orthogonalthereto. In some embodiments the 2D FOV of waveguide 120 may be definedby a 1D FOV in the (y,z) plane and a 1D FOV in the (x,z) plane, whichmay also be referred to as the vertical and horizontal FOVs,respectively.

FIG. 2 schematically illustrates the coupling of light of a wavelength λinto a waveguide 210 for a range of incidence angles α. Waveguide 210may represent the waveguide 120 of display 100, or any waveguide of awaveguide stack that may be used in place of the waveguide 120.Waveguide 210 may be a slab waveguide with a waveguide body that may befor example in the form of, or include, a substrate 205. The substrate205 may be a thin plate of an optical material that is transparent invisible light, such as glass or suitable plastic or polymer asnon-limiting examples. Main outer faces 211, 212 of waveguide 210,through which image light may enter or leave the waveguide, may benominally parallel to each other. The refractive index n of thesubstrate material may be greater than that of surrounding media, and insome embodiments may be for example in the range of 1.4 to 2.0. In someembodiments, high-index materials may be used for the substrate 205 or aportion thereof. In some embodiments these materials may have an indexof refraction n greater than about 2.2. In some embodiments thesematerials may have an index of refraction n greater than about 2.3. Insome embodiments these materials may have an index of refraction ngreater than about 2.5. Non-limiting examples of such materials arelithium niobate (LiNbO3), titanium dioxide (Ti02), gallium nitride(GaN), aluminum nitride (AlN), silicon carbide (SiC), CVD diamond, zincsulfide (ZnS).

An in-coupler 230 may be provided in an input area 203 of the waveguide210 and may be in the form of one or more diffraction gratings. Anout-coupler 240, which may also be in the form of one or morediffraction gratings, may be disposed in an output area 209 of thewaveguide, and may be laterally offset from the in-coupler 230, forexample along the y-axis. In the illustrated embodiment the out-coupler240 is located at the same face 211 of the waveguide 210 as thein-coupler 130, but in other embodiments it may be located at theopposite face 212 of the waveguide. Some embodiments may have two inputgratings that may be disposed at main outer faces 211, 212 of thewaveguide, and/or two output gratings that may be disposed at main outerfaces 211, 212 of the waveguide, or superimposed at a same face or at asame plane within the waveguide. The gratings embodying couplers 230,240 may be any suitable diffraction gratings, including volume andsurface-relief gratings, such as for example blaze gratings. Thegratings may also be volume holographic gratings. In some embodimentsthey may be formed in the material of the waveguide itself In someembodiments they may be fabricated in a different material or materialsthat may be affixed to a face or faces of the waveguide at desiredlocations.

The in-coupler 230 may be configured to support an input FOV 234, whichmay also be referred to herein as the acceptance angle. The input FOV234, which depends on wavelength, defines a range of angles of incidenceα for which the light incident upon the in-coupler 230 is coupled intothe waveguide and propagates toward the out-coupler 240. In the contextof this specification, “coupled into the waveguide” means coupled intothe guided modes of the waveguide or modes that have suitably lowradiation loss. Light coupled into the waveguide that experiences totalinternal reflection (TIR) upon the waveguide's outer surfaces 211 and212 may propagate within the waveguide with suitably low attenuationuntil it is redirected by an out-coupler. Thus waveguide 210 may traplight of a particular wavelength λ by means of TIR, and guide thetrapped light toward the out-coupler 240, provided that the angle ofincidence of the light upon the in-coupler 230 from the outside of thewaveguide is within the input FOV 234 of the waveguide 210. The inputFOV 234 of the waveguide is determined at least in part by a pitch p ofthe in-coupler grating 230 and by the refractive index n of thewaveguide. For a given grating pitch p, the first-order diffractionangle β of the light incident upon the grating 230 from the air at anangle of incidence a in the (y, z) plane may be found from a diffractionequation (1):

n·sin(β)−sin(α)=λ/p.   (1)

Here the angle of incidence α and the diffraction angle β are positiveif the corresponding wavevectors have components directed toward theout-coupler 240. Diffraction angle β defines the angle of propagation ofthe diffracted ray of the image light in the waveguide, and may also bereferred to herein as the propagation angle. Equation (1) may be easilymodified for embodiments in which light enters the waveguide 210 from amaterial with refractive index n_(c)>1. Equation (1) holds for rays ofimage light with a plane of incidence normal to the groves of thein-coupler grating, i.e. when the plane of incidence of image lightincludes the grating vector of the in-coupler. In the illustratedexample, the grating vector of the in-coupler may be directed along they-axis.

Light experiencing TIR in a waveguide or a layer may be referred toherein as the in-coupled light or trapped light. The TIR condition forthe diffracted light within the waveguide may be defined by equation(2):

n·sin(β)≥1,   (2)

where the equality corresponds to a TIR angle β_(c)=asin(1/n). The inputFOV 234 of waveguide 210 spans between a first FOV angle of incidence α₁and a second FOV angle of incidence α₂, which may be referred to hereinas the FOV angles. The first FOV angle of incidence α₁ corresponding tothe right-most incident ray 111 b in FIG. 2A may be defined by the TIRangle β_(c) of the in-coupled light, i.e. light trapped within thewaveguide:

$\begin{matrix}{{\alpha_{1} = {a\;{\sin( {1 - \frac{\lambda}{p}} )}}},} & (3)\end{matrix}$

The second FOV angle of incidence α₂, corresponding to the left-mostincident ray 111 a in FIG. 2A, may be defined by a limitation on amaximum angle β_(max) of the in-coupled light:

$\begin{matrix}{{\alpha_{2} = {a\;{\sin( {{n \cdot {\sin( \beta_{\max} )}} - \frac{\lambda}{p}} )}}},} & (4)\end{matrix}$

The width w=|α₁−α₂| of the input 1D FOV of the waveguide 210 at aparticular wavelength can be estimated from equations (3) and (4).Generally the input FOV of a waveguide increases as the refractive indexof the waveguide increases relative to that of the surrounding media. Byway of example, for a substrate of index n surrounded by air and forβ_(max)=75°, λ/p=1.3, the width w of the input 1D FOV of the waveguidefor monochromatic light may be about 26° for n=1.5, about 43° for n=1.8,and about 107° for n=2.4.

As can be seen from equations (3) and (4), the input FOV 234 ofwaveguide 210 is a function of the wavelength λ of input light, so thatthe input FOV 234 shifts its position in the angle space as thewavelength changes; for example, it shifts towards the out-coupler 240as the wavelength increases. Thus it can be challenging to provide asufficiently wide FOV for polychromatic image light with a singlewaveguide.

Referring to FIG. 3, light coupled into the waveguide 210 by thein-coupler 230 propagates in the waveguide toward the out-coupler 240.The out-coupler 240 is configured to re-direct at least a portion of thein-coupled light out of the waveguide 210 at an angle or angles withinan output FOV 244 of the waveguide, which is defined in part by theout-coupler 240. An overall FOV of the waveguide, i.e. the range ofincidence angles a that may be conveyed to the viewer by the waveguide,may be affected by both the in-coupler 230 and the out-coupler 240.

In some embodiments the gratings embodying the in-coupler 230 and theout-coupler 240 may be configured so that the vector sum of theirgrating vectors g_(i) is equal to substantially zero:

|Σg _(i)|≅0.   (5)

Here the summation in the left hand side (LHS) of equation (5) isperformed over grating vectors g_(i) of all gratings that cooperate todiffract the input light traversing the waveguide, including the one ormore gratings of the in-coupler 230, and the one or more gratings of theout-coupler 230. A grating vector g_(i) is a vector that is directednormally to the equal-phase planes of the grating, i.e. its “grooves”,and which magnitude is inversely proportional to the grating pitch p,|g_(i)|=2π/p. Under conditions of equation (5), rays of the image lightexit the waveguide by means of the out-coupler 240 at the same angle atwhich they entered the in-coupler 230, provided that the waveguide 210is an ideal slab waveguide with parallel outer faces 211, 212, and theFOV of the waveguide is defined by its input FOV. In practicalimplementations the equation (5) will hold with some accuracy, within anerror threshold that may be allowed for a particular display system. Inan example embodiment with a single 1D input grating and a 1D outputgrating, the grating pitch of the out-coupler 240 may be substantiallyequal to the grating pitch of the in-coupler 230.

FIG. 4 illustrates an embodiment in which the out-coupler 240 includestwo diffraction gratings 241, 242, which may be disposed for example atmain outer faces of the waveguide. The diffraction gratings 241 and 242may be configured so that the in-coupled light 211 a exits the waveguideas output light 221 after being sequentially diffracted once by each ofthese gratings. In some embodiments, the grating vectors g₁ and g₂ ofthe diffraction gratings 241, 242 may be directed at an angle to eachother. In at least some embodiments they may be selected so that(g₀+g₁+g₂)=0, where g₀ is the grating vector of the in-coupler 230.

FIG. 5 illustrates, in a plan view, a display waveguide 410 with anin-coupler 430 disposed in an input area 403 of the waveguide, and anout-coupler 440 disposed in an output area 407 of the waveguide. Thein-coupler 430 may be in the form of an input diffraction grating with agrating vector g₀ directed generally toward the out-coupler 440. Theout-coupler 440 is comprised of two output linear diffraction gratings441 and 442 with grating vectors g₁ and g₂ oriented at an angle to eachother. In some embodiments gratings 441 and 442 may be lineardiffraction gratings formed at main outer faces of the waveguide. Insome embodiments they may be superimposed upon each other at either faceof the waveguide, or in the volume thereof, to form a 2D grating. Light401 incident upon the in-coupler 430 within a FOV of the waveguide maybe coupled by the in-coupler 430 into the waveguide to propagate towardthe out-coupler 440, expanding in size in the plane of the waveguide, asillustrated by in-coupled rays 411 a and 411 b. The gratings 441, 442are configured so that consecutive diffractions off each of themre-directs the in-coupled light out of the waveguide. Rays 411 a may berays of in-coupled light that, upon entering the output area 407 of thewaveguide where the out-coupler 440 is located, are first diffracted bythe first grating 441, and then are diffracted out of the waveguide bythe second grating 442 after propagating some distance within thewaveguide. Rays 411 b may be rays of the in-coupled light that are firstdiffracted by the second grating 442, and then are diffracted out of thewaveguide by the first grating 441. An exit pupil 450 of the waveguide,which may also be referred to as the eyebox projection area 450, isindicated where the out-coupled light has optimal characteristics forviewing, for example where it has desired dimensions. The exit pupil 450may be located at some distance from the in-coupler 430.

FIG. 6A illustrates aspects of operation of the display waveguide 410with reference to a (K_(x), K_(y)) plane, where K_(x) and K_(y) denote xand y coordinates of a projection of a normalized k-vector K=k·2π/λ ofthe image light upon the plane of the waveguide:

K _(x) =n sin(θ_(x)), and K _(y) =n sin(θ_(y)).   (6)

Here n is the refractive index of the substrate where in-coupled lightis propagating, and the angles θ_(x) and η_(y) define the direction oflight propagation in the plane of the waveguide (x,y) in projection onthe x-axis and y-axis, respectively. These angles may also represent thecoordinates of angle space in which a 2D FOV of the waveguide may bedefined. The (K_(x), K_(y)) plane may be referred to herein as theK-space, and the normalized wavevector K=(K_(x), K_(y)) as the in-planeK-vector or simply as the K-vector.

In the K-space, the in-coupled light may be graphically represented by aTIR ring 500. The TIR ring 500 is an area of the K-space bounded by aTIR circle 501 and a maximum-angle circle 502, both circles centered atK₀=(0, 0) corresponding to the normal incidence upon the waveguide. TheTIR circle 501 corresponds to the TIR angle β_(c). The maximum-anglecircle 502 corresponds to a maximum propagation angle β_(max) forin-coupled light. States within the TIR circle 501 represent uncoupledlight, i.e. the in-coming light that is incident upon the in-coupler430, or the light coupled out of the waveguide by the out-couplergratings 441 and/or 442. With the normalization, the radius r_(TIR) ofthe TIR circle 501 and the radius r_(max) of the outer circle 502 may bedefined by the following equations:

r_(TIR)=1, r _(max) =n·sin(β_(max))   (7)

The greater the refractive index n, the wider is the TIR ring 500 andthe broader is the angular range of input light of a wavelength λ thatcan be coupled into the waveguide.

Arrows labeled g₀, g₁, and g₂ in FIG. 6A represent the normalizedgrating vectors of the in-coupler 430, the first out-coupler grating441, and the second out-coupler grating 442, respectively, for aparticular wavelength λ, with grating lengths g_(i)=λ/p_(i), where p_(i)is the pitch of i-th grating, i=0, 1, or 2. In the figure these gratingvectors form two closed triangles describing two possible paths in theK-space along which the incoming light may return to the same K-spacestate after being diffracted once by each of the three gratings, therebypreserving the direction of propagation in the angle space from theinput to the output of the waveguide. Each diffraction may berepresented as a shift in the (K_(x), K_(y)) plane by a correspondinggrating vector. Areas 520, 530 in combination represent the FOV of thewaveguide in the (K_(x), K_(y)) plane, and may be referred to as thefirst and second partial FOV areas, respectively. They are defined bythe in-coupler and out-coupler gratings and the refractive index of thewaveguide, and represent all k-vectors of light stays trapped within thewaveguide (the TIR ring 500) after consecutive diffractions upon theinput grating 430 and one of the output gratings 441, 442, and returnsto a same (K_(x),K_(y)) location in the interior of the TIR circle 501after a subsequent diffraction upon the other of the two outputgratings. The first partial FOV area 520 may be determined byidentifying all (K_(g), K_(y)) states which are imaged to itself byconsecutive diffractions upon the input grating 430, the first outputgrating 441, and the second output grating 442, each of which may berepresented as a shift in the (K_(x), K_(y)) plane by a correspondinggrating vector. The second partial FOV area 530 may be determined byidentifying all (K_(x), K_(y)) states which are imaged to itself byconsecutive diffractions upon the input grating 430, the second outputgrating 442, and the first output grating 441.

FIG. 6B illustrates the first and second partial FOVs 520, 530 in a 2Dangle space, with the horizontal and vertical axes representing theangles of incidence θ_(x) and74 _(y) of input light in the x-axis andy-axis directions, respectively, both in degrees. The (0,0) pointcorresponds to normal incidence upon the in-coupler. In combination,partial FOVs 520, 530 define a full FOV 550 of the waveguide at thewavelength λ, which encompasses all incident rays of input light of theselected color or wavelength that may be conveyed to a user. Arectangular area 555 which fits within the full FOV 550 may define amonochromatic FOV of the waveguide that may be useful in a display.

The position, size, and shape of each partial FOV 520, 530 in the anglespace, and thus the full 2D FOV of the waveguide, depends on thewavelength λ of the input light, on the ratios of pitches p₀, p₁, and p₂of the input and output gratings to the wavelength of incoming light λ,and on the relative orientation of the gratings. Thus, the 2D FOV of thewaveguide may be suitably shaped and positioned in the angle space for aparticular color channel or channels by selecting the pitch sizes andthe relative orientation of the gratings. In some embodiments ofwaveguide 410, the output gratings 441, 442 may have the same pitch,p₁=p₂ and be symmetrically oriented relative to the input grating. Insuch embodiments the grating vectors g₁, g₂ of the first and secondoutput gratings may be oriented at angles of +\−ϕ relative to thegrating vector g₀ of the in-coupler. By way of non-limiting example, thegrating orientation angle ϕ may be in the range of 50 to 70 degrees, forexample 60 to 66 degrees, and may depend on the refractive index of thewaveguide. FIG. 6B illustrates the FOV of an example waveguide with therefractive index n=1.8, ϕ≅60°, and p1=p₂=p₃=p, with p/λ selected tocenter the FOV 555 at normal incidence.

In some embodiments, a single waveguide formed of an opticallytransparent high-index substrate may be used in a display system toconvey multiple color channels of RGB light from an image source to aviewing area of a waveguide display, such as an eyebox of a, NED. Insome embodiments the same input and output gratings may be used for atleast two color channels of the image light, for example for at leasttwo of the Red, Green, and Blue RGB color channels, or for all three RGBcolor channels. The desirability of having a high refractive index n maybe understood by noting that in the wavelength-normalized K-space thelength of each grating vector scales with the wavelength, i.e.g_(i)=λ/p_(i), where p_(i) is the pitch of the 1-th grating, i=0, 1, or2. As the width of the TIR ring 500 is proportional to the refractiveindex n, greater values of the refractive index enable broaderpolychromatic FOV, i.e. the common FOV that is shared by two or morecolor channels of the image light.

FIG. 7 illustrates the wavelength-scaled grating vectors 531, 532 of thein-coupler 430 for two different wavelength λ₁ and λ₂>λ₁ in the K-spacediagram of FIG. 6A. In the illustrated example the grating vector of thein-coupler is directed along the y-axis. The normalized grating vector531 for the first wavelength λ₁ has a length λ₁/p₀, while the longernormalized grating vector 532 has a length of λ_(2/)p₀. The firstwavelength λ₁ may be, for example, a peak wavelength of the blue colorchannel, while the second wavelength λ₂ may be, for example, a peakwavelength of the red color channel.

In some embodiments a maximum y-axis width (2α) of a symmetrical FOVshared by the two wavelengths, FOV_(y)=(−α,+α), may be estimated fromequations (8) and (9).

1+sin(α)=λ₁ /p ₀   (8)

n·sin(β_(maz))−sin(α)=λ₂ /p ₀   (9)

Equation (8) represents a condition that the shorter of the gratingvectors 531, 532 is long enough to reach the TIR circle 501 from a stateA1 of the FOV that is farthest from the TIR circle 501 in the directionof the grating vector. Equation (9) represents a condition that aK-state A2 at an opposite end of the FOV is far enough from the outerboundary 502 of the TIR ring 500 that the longer of the two gratingvectors 531, 532 doesn't extend beyond it. These conditions provide anestimate for the pitch p₀ of the in-coupler grating (equation (10)) andan estimate of the corresponding half-width α of the common FOV of thetwo wavelengths along the y-axis (equation 11):

$\begin{matrix}{p_{0} = \frac{\lambda_{2} + \lambda_{1}}{{n \cdot {\sin( \beta_{\max} )}} + 1}} & (10) \\{\alpha = {a\;{\sin( \frac{{\lambda_{1} \cdot n \cdot {\sin( \beta_{\max} )}} - \lambda_{2}}{\lambda_{2} + \lambda_{1}} )}}} & (11)\end{matrix}$

An estimated width 2α of the shared 1D FOV for wavelengths λ₁ and λ₂increases as the refractive index n of the waveguide increases above aminimum value n_(min), which in some embodiments may be estimated asn_(min)=λ₂/λ₁ sin(β_(max)). By way of example the longer wavelength λ₂may correspond to red light, with the wavelength e.g. of 635 nm, whilethe shorter wavelength λ₁ may correspond to blue light, with thewavelength e.g. of 465 nm, resulting in a minimal value of n of about1.4 for a waveguide configured to transmit all three color channels ofRGB light. According to Equation (11), in one embodiment an estimatedwidth 2α of a symmetrical 1D FOV of a single one-layer waveguide thatmay be shared by all three channels of RGB light may be about 30 degreesfor n=2.0, about 40 degrees for n=2.2, and about 63 degrees for n=2.6.

Equation (11) provides an estimate of a 1D FOV that is centered atnormal incidence and may be supported by a single waveguide forpolychromatic light with wavelengths from λ₁ to λ₂. A 2D FOV that thewaveguide supports for polychromatic light at its output, e.g. at theeyebox, may further depend on the out-coupler, such as the number andconfiguration of the output gratings.

In some embodiments, a single waveguide made of optically transparenthigh-index material with the refractive index of about 2.3, orpreferably 2.4 or greater may be used in a display system to convey RGBlight from an image source to an eyebox of a NED. In some embodiments, aNED may transmit image light with a single waveguide made of opticallytransparent high-index material with the refractive index of at least2.5. In some embodiments, a NED may transmit image light with a singlewaveguide made of optically transparent high-index material with therefractive index of at least 2.6.

Referring now to FIG. 8, rays of different color channels coupled into awaveguide by a same grating will propagate in the waveguide at differentangles, with the difference the greater the bigger is the spectraldistance between the color channel. FIG. 8 schematically illustrates thepropagation of ray 611B of a blue color channel and ray 611R of a redcolor channel of input RGB light 611 in a display waveguide 600. Thedisplay waveguide 600 may be formed of a waveguide body 610 such as ahigh-index substrate, an input coupler 630, and an output coupler 640.The input coupler 630 and the output coupler 640 may be for example asdescribed above with reference to couplers 230, 240 of FIGS. 3, 4 orcouplers 430, 440 of FIG. 5. In the illustrated embodiment the inputcoupler 630 is in the form of a diffraction grating of a pitch p₀.Diffraction on this grating disperses the incident RGB light 611according to the wavelength. As a result, ray 611R of the red colorchannel may propagate in the substrate 610 at a significantly greaterangle β_(R) than the propagation angle β_(B) of ray 611B of the bluecolor channel. This divergence of color channels in the substrate 610 ofthe display waveguide 600 may necessitate a more complex design of theout-coupler 640. Indeed, ray 611B propagating at a smaller angle mayexperience a greater number of bounces off the outer faces of thewaveguide than the ray 611R that propagates at a greater angle. Inembodiments where the output gratings are formed at the outer faces ofthe waveguide, ray 611B will also experience a greater number ofinteractions with the output gratings. A similar or even greaterdiscrepancy in the grating-ray interactions arises for image rays of asame wavelength at the edges of the input FOV, such as rays 111 a and111 b shown in FIG. 2. Therefore, maintaining color and brightnessuniformity across the FOV may require output gratings with a strongangular dependence of the grating efficiency. The gratings may have tobe specifically designed to compensate for the difference in the numberof waveguide “bounces”, or ray-grating interactions, per unit length ofthe output area of the waveguide for image rays of different colorsand/or different angles of incidence. This may significantly complicatethe grating design.

Referring to FIG. 9A, there is illustrated an example two-layer displaywaveguide 700 according to an embodiment of the present disclosure. Thedisplay waveguide 700 addresses the angle difference issue by directingat least a portion of the longer-wavelength color channel to propagatein a higher-index material. The two-layer display waveguide 700, whichmay be referred to hereinafter simply as waveguide 700, may be anembodiment of waveguide 120 or waveguide 410 described above. It has awaveguide body 707 that includes a high-index (HI) layer 710 supportedby a substrate 720, which may have a relatively lower index ofrefraction. The HI layer 710 may also be referred to herein as the toplayer 710. The substrate 720 plays the role of a second, or lower, layerof the two-layer waveguide 700. Both the HI layer 710 and the substrate720 may be made of optically transparent materials having suitably lowabsorption coefficients. The HI layer 710 may be disposed over thesubstrate 720 without an airgap between them to form a contiguouswaveguide body. The term “high-index layer” as used herein may refer toa layer with a refractive index n₁ that is greater than the refractiveindex n₂ of the substrate 720, n₁>n₂. In one embodiment, nz may be inthe range of 1.5 to 2.0, and n₁ may be in the range of 2.2 to 2.6 orgreater, with the upper limit that may be defined by availability ofsuitable materials. The difference Δn=(n₁−n₂) between the refractiveindices of the layers may be greater than 0.3 in at least someembodiments. In some embodiments the index difference Δn may be in therange from 0.3 to 1. In some embodiments An may be greater than 0.4. Insome embodiments Δn may be greater than 0.5.

than about 0.4. Examples of possible material that may be used for theHI layer 710 include LiNbO3, Ti02, GaN, AlN, SiC, CVD diamond, ZnS. Thethickness of the HI layer 710 and the substrate 720 may vary dependingon their indices and/or design goals. By way of non-limiting example,the thickness d₁ of the HI layer 710 may be in the range of 150 to 400μm, for example about 300 μm. The substrate thickness d₂ may be, forexample, between 200 and 600 μm. Embodiments with the layer andsubstrate thickness outside of these ranges may also be envisioned.

Continuing to refer to FIG. 9A, waveguide 700 may include an in-coupler730 for coupling image light 711 into the waveguide, and out-coupler 740for coupling in-coupled image light out of the waveguide toward aneyebox 754. In one embodiment the in-coupler 730 may be in the form of,or include, an input grating. In one embodiment the in-coupler 730 maybe in the form of, or include, a prism. An input grating implementingthe in-coupler 730 may be disposed so as to inject light directly intothe HI layer 710, and may be disposed in or upon the HI layer 710. Insome embodiments an input grating may also be formed at an interface 702between the HI layer 710 and substrate 720. In some embodiments morethan one input grating may be provided. For example, in embodiments withtwo or more input gratings, at least one such grating may be disposed inthe HI layer 710 or at a surface thereof. The out-coupler 740 may belaterally offset from the in-coupler 730, for example along the y-axisin the direction of the grating vector of the in-coupler. Theout-coupler 740 may be in the form of two or more diffraction gratings,at least some of which configured to diffract light propagating in theHI layer 710. In one embodiment the out-coupler 740 may include a firstgrating 741, which may be disposed at a surface 701 of the HI layer 710,and a second grating 742, which may be disposed at the interface 702between the substrate 720 and the HI layer 710. In some embodiments oneor both of these gratings 741, 742 may be disposed within the HI layer710. In some embodiments a pair of optional output gratings 743, 744 maybe disposed within the substrate 720 and/or at a surface thereof, asillustrated in FIG. 9B. The surfaces 701, 703 may define two main outerfaces of the waveguide body, which bound the in-coupled image lightwithin the waveguide body by TIR as it propagates from the in-coupler tothe out-coupler, and through which image light may enter and/or exitsthe waveguide.

Image light 711 may include a first color channel and a second colorchannel, with the second color channel comprising longer wavelengthsthan the first color channel. The first color channel may be indicatedwith a letter “B” and referred to as the blue color channel or bluelight, and the second color channel may be indicated with a letter “R”and referred to as the red color channel or the red light. In someembodiments the first color channel and the second color channel maycorrespond to the “B” and “R” color channels of RGB light. The HI layer710 may be configured to trap at least a portion of the red colorchannel of image light 711 by means of TIR at the interface 702 withsubstrate 720, while allowing the blue color channel of the image lightto propagate into the substrate 720, where it can be trapped by TIR atthe second outer surface 703 thereof. This is schematically illustratedby ray 711R of the red color channel of image light 711 being trapped inthe HI layer 710, and ray 711B of the blue color channel of image light711 propagating into the substrate 720 and experiencing TIR at thesurface 703 thereof. Due to the higher refractive index of HI layer 710relative to substrate 720, the propagation angle β₁ of the red ray 711Rin the HI layer 710 and the propagation angle of the blue ray 711Bwithin substrate 720 may be substantially closer to each other thantheir propagation angles in the HI layer 710 or the propagation anglesof rays 611R and 611B in the waveguide 600 of FIG. 8. In someembodiments the thickness d₁ of layer 710 and the thickness d₂ ofsubstrate 720 may be selected so as to substantially equalize the numberof bounces that red ray 711R and blue ray 711B experience on their wayto the out-coupler 740. In some embodiments the thickness d₁ of layer710 and the thickness d₂ of substrate 720 may be selected so as tosubstantially equalize the number of interactions with the out-couplergratings 741, 742 that the red ray 711R and the blue ray 711B experiencewhen propagating in the output area of the waveguide's body where theout-coupler 740 is disposed.

The coupling of image light 711 of wavelength λ in the HI layer 710 bythe input grating 730 may be described using equation (1) with n=n₁ andp=p₀, where p₀ is the pitch of the input grating 730. A TIR condition onthe interface 702 may be expressed as

n ₁ sin(β₁)≥n ₂   (12)

From equations (1) and (12), a first specific angle of incidence α₁₂ ofimage light 711 upon the input grating 730, i.e. the angle of incidenceat which the image light of wavelength λ experiences TIR at interface702 may be estimated from the following equation (13):

$\begin{matrix}{{\sin( \alpha_{12} )} = {n_{2} - \frac{\lambda}{p_{0}}}} & (13)\end{matrix}$

Rays of image light 711 that are incident upon the waveguide at smallerangles, α<α₁₂, may propagate into the substrate 720 and may experienceTIR at the outer surface 703 thereof. Rays incident at a slightlygreater angle than α₁₂ will be partially reflected back into the HIlayer 710 and partially propagate into the substrate 720 at a “glancing”angle as illustrated at 713, with the reflected fraction the greater thecloser to the TIR condition at the interface 702, and the transmittedfraction vanishing at the TIR condition.

A second specific angle of incidence α₂₃ upon waveguide 700, i.e. thesmallest angle of incidence for which the image light of wavelength λexperiences TIR at the outer surface 703, may be estimated from thefollowing equation (14):

$\begin{matrix}{{\sin( \alpha_{23} )} = {1 - \frac{\lambda}{p_{0}}}} & (14)\end{matrix}$

A limitation on a maximum propagation angle ϵ_(2max) of the in-coupledlight in substrate 720 yields a third specific angle of incidence α₃,which may be estimated from the following equation (15):

$\begin{matrix}{{{\sin( \alpha_{3} )} = {{n_{2} \cdot {\sin( \beta_{\max} )}} - \frac{\lambda}{p_{0}}}},} & (15)\end{matrix}$

A limitation on a maximum propagation angle β_(1max) of the lighttrapped in the HI layer 710 yields a fourth specific angle of incidenceα₄, which may be estimated from the following equation (16):

$\begin{matrix}{{{\sin( \alpha_{4} )} = {{n_{1} \cdot {\sin( \beta_{\max} )}} - \frac{\lambda}{p_{0}}}},} & (16)\end{matrix}$

FIG. 9B illustrates the propagation of example rays 716 and 717 of imagelight 711 of a wavelength λ in the multi-layer waveguide 700. Rays 716and 717 may be received from the opposite edges of an input FOV 715 ofthe waveguide. The input FOV 715 defines, for a particular plane ofincidence, the angular range of incidence for which image light 711 maybe coupled into the waveguide 700 by the input grating 730. Image lightreceived from a portion of the input FOV 715 that corresponds torelatively greater propagation angles β₁>α₁₂ in the IH layer 710, asexemplified by ray 716, may be trapped in that layer, and propagatetoward the out-coupler 740 guided by TIR at the top outer surface 701and the interface 702. Image light of the same wavelength received fromthe rest of the input FOV 715, as exemplified by the second ray 717, mayat least partially pass through the interface 702 into substrate 720 andpropagate toward the out-coupler 740 guided at least partially by TIR atthe outer surfaces 701, 703 of the waveguide. By confining image lightreceived from a portion of the FOV 715 in a high-index region or portionof the waveguide's thickness, such as the HI layer 710, the discrepancyin the grating-light interactions between different portions of the FOVmay be advantageously reduced.

FIG. 10 shows a K-state diagram illustrating the operation of an exampleembodiment of waveguide 700 in the normalized K-space (K_(x), K_(y)).Similarly to diagrams of FIGS. 6A and 7, the interior of a first TIRcircle 801 represents the in-plane K-vectors of uncoupled light. Theinterior of a first TIR ring 805 represents in-coupled light that atleast partially passes the layer-substrate interface 702 and experiencesTIR at the outer surface 703 of substrate 720. The first TIR ring 805 isbounded by the first TIR circle 801 of radius r₁=1 and a firstmaximum-angle circle 802 of radius r₂=n₂·sin(β_(2max)). The interior ofa second TIR ring 806 represents in-coupled light that is trapped in theHI layer 710 and experiences TIR at the interface 702 with substrate720, with the angle of incidence upon it less than β_(1max), i.e.excluding “glancing” rays in the HI layer 710. The second TIR ring 806is bounded by a second TIR circle 803 of radius r₃=n₂ and a secondmaximum-angle circle 804 of radius r₄ 32 n₁·sin(β_(1max)). Thein-coupler 730 is represented by the wavelength-scaled grating vectorg₈₀ of length g_(λ)=λ/p₀ that couples an uncoupled K-state inside thefirst TIR circle 801 to a coupled K-state shifted by the grating vectorg₈₀ .

In FIG. 10, the operation of waveguide 700 for a selected wavelength λis illustrated by way of example with reference to two differentK-states of incident light. State “A” in the center (0,0) of the K-spacecorresponds to a normal incidence of the image light upon the waveguide.State “B” corresponds to image light with a non-zero angle of incidenceα_(B) upon the waveguide, 0<α_(B)<90; state “B” lies at a distanced_(B)=|sin(α_(B))| from the K-space center (0,0). In the illustratedexample, the in-coupler 730 transfers state “A” into a state within thefirst TIR ring 805, and transfers state “B” into a state within thesecond TIR ring 806. Hence normally incident light of wavelength λ willat least partially pass through the interface 702 and bounce off thesubstrate's outer surface 703 by TIR. Image light of the same wavelengththat is incident upon the waveguide at angle α_(B), which in the absenceof the HI layer 710 would pass through the waveguide without beingcoupled, will be trapped within the HI layer 710. Thus, the addition ofthe HI layer 710 expands the k-space of in-coupled light, therebybroadening the FOV that waveguide 700 is able to support.

FIG. 11 illustrates by way of example the specific angles of incidenceestimated from equations (13), (14), and (16) for wavelengths of the redand blue color channels in dependence on the pitch p of the in-couplergrating. An embodiment of waveguide 700 with n₁=2.6 and n₂=1.9 isassumed. Wavelengths λ_(R)=640 nm and λ_(B)=465 nm are assumed for thered and blue color channels, respectively. Solid lines relate to theblue color channel, dotted lines relate to the red color channel. Thefirst specific angle of incidence corresponding to the onset of TIR atthe HI layer-substrate interface 702 is shown at 810R for the Red colorchannel and at 810B for the blue color channel. The second specificangle of incidence corresponding to the onset of TIR at the outersurface 703 is shown at 811R for the Red color channel and at 811B forthe blue color channel. The third specific angle of incidence defined bythe limitation on the maximum propagation angle β_(1max) in the HI layeris shown at 813R for the Red color channel and at 813B for the bluecolor channel. For each color channel, curves 811x and 813x bound therange of incidence angles that can be coupled into the waveguide, whilecurves 810x and 813x bound the range of incidence angles that can betrapped in the HI layer of the waveguide; here “x” stands for either “R”or “B”. Curves 811B and 813R bound the range 815 of incidence anglesthat can be coupled into the waveguide for both the red and bluewavelengths, thereby defining the width Θ of the input 1D FOV of thewaveguide for RGB light. In the illustrated example, light of the redcolor channel may be trapped within the HI layer 710 if received in afirst portion 821 of the input 1D FOV 815, and may propagate part of thetime in the substrate 720 if received in a second, smaller portion 822of the input 1D FOV 815. In this example, for embodiments with p smallerthan about 300 nm no in-coupled light of the blue wavelength is trappedin the HI layer 710 by TIR. In embodiments with p greater than about 300nm, light of the blue wavelength received in a small portion 831 of theinput FOV 815 may be trapped in the HI layer 710. In embodiments withsmaller refractive index n₂ of the substrate, propagation in the HIlayer 710 may support a greater portion of the 1D FOV for both the redand blue color channels. In embodiments with a smaller refractive indexn₁ of the HI layer 710, a greater portion of the 1D FOV for both the redand blue color channels may be supported by propagation in the substrate720. For the green color channel, the fraction of light trapped in theHI layer will be between the corresponding fraction for the red and bluecolor channels. In at least some embodiments the HI layer 710 maysupport at least 50% of the angular FOV width Θ of at least one of thecolor channels of the image light.

Light propagating in the HI layer 710 may be diffracted out of thewaveguide by out-coupler gratings 741 and 742, which may be generallydisposed anywhere within the HI layer or at a surface thereof. Theout-coupler gratings 741 and 742 may be configured so that the sum(₁+g₂) of their grating vectors g₁ and g₂ equals to (−g₀), where g₀ isthe grating vector of the in-coupler 730, so that successive diffractionfrom these two gratings de-couples the in-coupled light from thewaveguide in the direction of its incidence upon the waveguide.In-coupled light that penetrates into the substrate 720 may beout-coupled from the waveguide by diffraction upon some combination oftwo or more of the first grating 741, the second grating 742, and anoptional third grating 743 when present. In some embodiments the thirdout-coupler grating 743 may be configured so that the vector sum of itsgrating vector g₃ with the grating vector g₁ of the first grating 741and the grating vector g₀ of the in-coupler 730 is substantially zero,so that a successive diffraction of the in-coupled light that propagatespartly in the substrate 720 from the first and third gratings 741, 743out-couples that light in the direction of its incidence upon thewaveguide. In some embodiment the second grating 742 and the thirdgrating 743 may have equal grating vectors g₃=g₂. In some embodimentsthe third grating 743 may be absent, and the in-coupled light thatpropagates partly in the substrate 720 may be out-coupled from thewaveguide by successive diffractions from the first grating 741 and thesecond grating 742. In some embodiments a fourth grating 744 with agrating vector g₄ may be provided to decouple the substrate-penetratingin-coupled light from the waveguide in cooperation with one of the firstgrating 741, the second grating 742, or the third grating 743. Thefourth grating 744 may be disposed for example at the outer substratesurface 703, where it may be superimposed or stacked with the thirdgrating 743. In some embodiments the grating vector g₄ of the fourthgrating 744 may be equal to the grating vector g₁ of the first grating741.

FIG. 12 illustrates a two-layer display waveguide 850 according to anembodiment. The two-layer display waveguide 850 may be an embodiment ofwaveguide 700 described above. The two-layer display waveguide 850includes substrate 852 of index n₂ as a first layer and an HI layer 851of index n₁>n₂ as a second layer. An in-coupler grating 830 is formed inor upon the HI layer 851 for example as a relief grating in a topsurface of the HI layer. An out-coupler 840 may include a first grating841, a second grating 842, and a third grating 83. Gratings 830, 841,842, and 843 may be embodiments of gratings 730, 741, 742, and 743 ofwaveguide 700 described above. In one embodiment the two-layer displaywaveguide 850 may be formed by a process that includes i) forming thesecond and third gratings 842, 843 at opposite faces of substrate 852,for example as relief gratings by etching, ii) optionally depositing athin capping layer over the second grating 842 to even the surface, andiii) depositing the HI layer 851 over the cap layer 855 or directly overthe substrate 852. In one embodiment the cap layer 855 may itselfinclude two or more layers. In one embodiment the substrate may be ofoptically transparent material with the refractive index in the range of1.4 and 2.0, such as for example glass, which may be suitably doped,plastic, or any other suitable optical material with low optical lossand the refractive index in a desired range, e.g. from 1.5 to 1.9 as anon-limiting example. The HI layer 851 may be of optically transparentmaterial with the refractive index that is greater than the refractiveindex of substrate 852, for example in the range of 2.2 and 2.6.Examples of higher-index material that may be suitable for the HI layer851 include those listed above with reference to waveguide 700.

FIGS. 9A, 9B, 12 illustrate embodiments where the out-coupler gratings741-743 are disposed at the opposite surfaces of the waveguide and atthe substrate-layer interface. Variations of this embodiment withdifferently placed and/or differently configured gratings may also beenvisioned. In at least some of those variations, the first and secondgratings may be configured so as to redirect light trapped in the HIlayer out of the waveguide. In some embodiments they may be formed inthe interior of the HI layer, for example as a holographic grating. Insome embodiments they may be superimposed in the interior of the HIlayer, at the surfaces of the HI layer, or at the interface of the HIlayer with the substrate. Similarly, the third grating that isconfigured to redirect in-coupled light not trapped in the HI layer maybe formed in the interior of the substrate or the HI layer, or it may besuperimposed with either one of the first and second gratings. In someembodiments, an out-coupler 740 may omit the third output grating.

The example multi-layer waveguide 700 illustrated in FIGS. 9A, 9B may bemodified, for example by changing the order of layers and/or adding morelayers. Embodiments may be envisioned where image light enters thewaveguide body through a substrate, and is then injected into an HIlayer for trapping therein by an input grating formed in the HI layer orat a surface thereof. For example, in a modified embodiments of FIG. 9A,image light 711 may enter waveguide 700 through an outer surface 703 ofsubstrate 720, and get coupled into the HI layer 710 by the inputgratings 730 configured to operate in reflection. In another embodiment,the input grating 730 may be formed within the HI layer 710 or at theinterface 702 of the HI layer 710 and substrate 720. In anotherembodiment an additional layer may be disposed over the HI layer, forexample with a lower refractive index than that of the HI layer.

Referring to FIG. 13, there is illustrated an example embodiment of amulti-layer waveguide 900 having a waveguide body 910 comprised of acenter layer 922 of thickness d₁, and two outer layers 921, 923 ofthicknesses d₀ and d₂, respectively. The multi-layer waveguide 900 maybe an embodiment of waveguides 120 and 410 described above. The threelayers 921, 922, 923 may be transparent to light of the operatingspectral range, such as visible light, and may form a contiguouswaveguide body 910. The center layer 922 may have a greater refractiveindex n₁ than refractive indices n₂ and n₃ of the two outer layers 921,923. The refractive indices n₂ and n₃ may be the same or different. Insome embodiment the refractive index difference between the center layerand the outer layers may be in the range of 0.3 to 1, or in someembodiments in the range of 0.4 to 0.7. In some embodiments the outerlayers 921, 923 may have the same refractive index n₂=n₃, althoughembodiments may be envisioned where the outer layers 921, 923 havediffering refractive indices. In some embodiments the refractive indexof one of the outer layers 921, 923 may be greater than of the other twolayers. An in-coupler 930 is configured to couple at least a portion ofthe image light into the HI layer 922 for propagating therein toward anout-coupler 340. The in-coupler 930 may include, or be in the form of,one or more input gratings that may be formed in the HI layer 922 or atan interface thereof with one of the outer layers; in the illustratedembodiment the in-coupler 930 is implemented as an input grating 930,which is disposed at the interface between the top outer layer 921 andthe HI layer 922 and operates in transmission. The out-coupler 940 isspatially offset from the in-coupler in the plane of the waveguide andis configured to de-couple image light propagating in the waveguidetoward a viewing area or eyebox 954. In some embodiments the out-coupler940 is formed with two output gratings 941 and 942 that may be formed atthe interfaces of the HI layer 922 with the outer layers 921, 922. Insome embodiments one or both of the output gratings 941 and 942 may beformed within the HI layer 92. In some embodiments the output gratings941 and 942 may be superimposed. The input grating 930 and the outputgratings 942, 942 may be configured so that their grating vectors sumsubstantially to zero, up to a fabrication accuracy. In some embodimentsthe out-coupler 940 may include additional output gratings, for exampleto diffract light propagating in at least one of the outer layers 921,923. In some embodiments the out-coupler 940 may include two additionalgratings 943, 944 for out-coupling light propagating in the first 921and the third 923 layers. In some embodiments these two additionalgratings may be disposed at the outer surfaces 991 and 994 of thewaveguide 900.

FIG. 13 schematically illustrates the propagation of two rays 951, 952of image light 911 in an embodiment of waveguide 910 with a centrallayer 922 having a greater refractive index n₁ and two outer layers ofsmaller refractive indices n₂ and n₃. The two rays 951 and 952 maycorrespond to opposite edges of a FOV of waveguide 900, with thepropagation angle β_(TIR) of ray 951 defined by the TIR condition at theouter faces 991, 994 of waveguide 900. In the absence of the central HIlayer 922, ray 952 would have propagated in the waveguide at anearly-glancing angle β_(max), resulting in a large discrepancy betweenthe two rays 951, 952 in the number of ray-grating interactions alongthe length of the out-coupler 940. By way of example, for n₂=n₃=1.9, the“glancing” ray 952 propagating in the waveguide at a maximum angleβ_(max)=75 deg would have about 6 times fewer bounces in the waveguideand interactions with the output grating than the TIR-limited ray 951,with the ratio increasing to 18 when considering even more “glancing”rays at β_(max)=85 deg.

The central HI layer 922 is configured to trap the “glancing” ray 952 byTIR within itself, generally in a way similar to that described abovewith reference to FIGS. 9A and 9B in relation to the HI layer 710. Bysuitably selecting the thicknesses d₀, d₁, and d₂ of the layers independences on their refractive indices, the difference in the number ofrelevant ray-grating interactions along the out-coupler 940 for rays 951and 952 at the opposite edges of the waveguide's FOV may besubstantially reduced, generally by approximately a thickness ratiod₁/d, where d is the total thickness of waveguide 910. This isillustrated in FIG. 13, schematically showing out-coupled rays 961(dotted lines) and 962 (dashed lines) produced by consecutivediffractions of rays 951 and 952, respectively, off the output gratings941 and 942, as image light corresponding to these rays propagate alongthe out-coupler 940. As can be seen in the figure, the distance betweenconsecutive diffraction events for rays 952 and 951 may be approximatelythe same, despite a large discrepancy of their propagation angles in theHI layer 922. By way of non-limiting example, the thickness d₁ of the HIlayer may be in the range of 20 to 50% of the total thickness d of thewaveguide.

Turning now to FIG. 14, there is illustrated an example multi-layerdisplay waveguide 980 having a contiguous waveguide body formed of aplurality of optically transparent layers 981 with varying refractiveindices. Waveguide 980, which may be a planar waveguide, is shown in thefigure in a cross-section by a plane perpendicular to the outer opticalfaces thereof. Waveguide 980 may be an embodiment of waveguides 120 and410 described above. In the illustrated embodiment five layers 981 areshown by way of example, although the number of layers in the waveguidemay be different in other embodiments. An insert at the right of FIG. 14is a plot illustrating the refractive indices nj of the layers, i=1, . .. , 5. In the illustrated example, the refractive index is greatest forthe middle layer, and decreases toward the outer faces of the waveguide;however embodiments where any of the other four layers has the greatestindex in the waveguide may also be envisioned. For example, in oneembodiment the two top layers or the two bottom layers may be omitted,so that the highest-index layer is adjacent to an outer surface of thewaveguide. The amplitude An of refractive index variation in thedirection of the waveguide's thickness, z-axis in the figure, may begreater than about 0.3, or preferably greater than about 0.4. Therefractive indices n_(i) and thicknesses d_(i) of the layers may beselected so as to confine more “gliding” rays within a narrower portionof the waveguide's body than rays coupled into the waveguide at smallerpropagation angles β. An in-coupler 983 may be disposed to couple atleast some portion of image light 911 into a high-index layer or layersof the waveguide; for example the in-coupler 983 may be implemented as adiffraction grating disposed at an interface of the central layerproximate to a source of image light (not shown). An out-coupler 984 maybe laterally offset from the in-coupler 983 and disposed to re-directin-coupled image light out of the waveguide, including light confined inthe high-index portion of the waveguide. For example the out-coupler 983may be implemented with two diffraction gratings that may be disposed atthe interfaces of the central layer with outer lower-index layers, asillustrated in FIG. 14. In some embodiments one or more additional inputgratings and/or additional output gratings may be provided.

Turning now to FIG. 15, there is illustrated an example gradient-indexdisplay waveguide 990 having a refractive index n continuously varyingin the direction of waveguide thickness, i.e. along the z-axis in thefigure. Waveguide 990, which may be a planar waveguide, is shown in thefigure in a cross-section by a plane perpendicular to the outer facesthereof. Waveguide 990 may be an embodiment of waveguides 120 and 410described above. An insert at the right of FIG. 15 is a plotillustrating the refractive index as a function n=n(z) of the “depth”coordinate z. In the illustrated example the waveguide is symmetrical inthe “depth” direction (z-axis) with the greatest refractive index in themiddle of the waveguide's body; however embodiments with an asymmetricalindex distribution of the refractive index relative to the outer facesof the waveguide may also be envisioned, some of which may have thelocation of the greatest index closer to one of the outer waveguidefaces than to the other, including when the highest index portion isnext to an outer surface of the waveguide. The amplitude Δn ofrefractive index variation in the direction of the waveguide'sthickness, z-axis in the figure, may be greater than about 0.3, orpreferably greater than about 0.4. The distribution of the refractiveindex n(z) as a function of the “depth” coordinate z may be optimized soas to confine more “gliding” rays within a narrower portion of thewaveguide's body than rays coupled into the waveguide at smallerpropagation angles β. In-coupler 1583 and out-coupler 1584 may beimplemented with diffraction gratings disposed in a higher-index portionof the waveguide to couple at least some of image light 911 into thehigher-index portion and to decouple the in-coupled light out of thewaveguide, generally as described above with reference to FIGS. 9A, 9B,12-14. For example the out-coupler 1584 may be implemented with twodiffraction gratings that may be disposed close to each other orsuper-imposed in a middle portion of the waveguide in the high-indexregion thereof. A gradient-index implementation may have some advantagesover a multi-layer implementation with abrupt steps in the refractiveindex at the layer interfaces. These advantages may including includeavoiding reflections of the image light off the refractive index steps,and a better homogenization of the pupil replication density acrossdifferent colors and angles of incidence of the image light. Agradient-index planar waveguide as described above may be implemented,for example, by layer fusion,

Embodiments described above with reference to FIGS. 9A-15 are examplesof a display waveguide for conveying image light in a display system,the waveguide having a contiguous waveguide body with two outer surfacesand a thickness therebetween, and a refractive index that varies in thedirection of the thickness. The waveguide further includes an inputcoupler disposed in the input area and configured to couple the imagelight into the waveguide body for propagating toward the output area,and an output coupler disposed in the output area and configured tocouple the image light out of the waveguide body for propagating towardan eyebox. The waveguide body has an input area and an output areaoffset therefrom, and is configured to guide image light from the inputarea toward the output area. In some embodiments, such as thosedescribed above with reference to FIGS. 13-15, the refractive index isgreater in a middle portion of the waveguide body between the outersurfaces than in portions of the waveguide body adjacent to the outersurfaces. In some embodiments, such as those described above withreference to FIGS. 9A-12, the refractive index is greater in a portionof the waveguide body adjacent to one of the outer surfaces thereof.

Embodiments described above implement a method for conveying image lightfrom an image light source to an eye box with a waveguide by selectivelyconfining a first portion of the image light in a smaller volume of thewaveguide's body than a second portion of the image light for equalizingconditions of their de-cooping out of the waveguides.

In some embodiments the method may include: directing the image lightonto an input area of a waveguide comprising two opposed outer surfacesand a high-index portion therebetween, the high-index portion extendingfrom the input area of the waveguide to an output area thereof, thehigh-index portion having a greater refractive index than a portion ofthe waveguide body adjacent to one of the opposed outer surfacesthereof; coupling the image light into the waveguide body at a range ofpropagation angles comprising a first propagation angle and a secondpropagation angle; and, propagating the image light in the waveguidefrom the input area to the output area so that first rays of the imagelight coupled into the waveguide at the first propagation angle areguided toward the output area by total internal refraction at theopposed outer surfaces of the waveguide, while second rays of the imagelight coupled into the waveguide at the second propagation anglepropagate toward the output area within the high-index portion of thewaveguide body.

In some embodiments the method may include: directing the image lightonto an input area of a waveguide comprising two opposed outer surfacesand a middle portion therebetween, the middle portion extending from theinput area of the waveguide to an output area thereof, the middleportion having a greater refractive index than portions of the waveguidebody adjacent to the opposed outer surfaces thereof; coupling the imagelight into the waveguide body at a range of propagation anglescomprising a first propagation angle and a second propagation angle;and, propagating the image light in the waveguide from the input area tothe output area so that first rays of the image light coupled into thewaveguide at the first propagation angle are guided toward the outputarea by total internal refraction at the opposed outer surfaces of thewaveguide, while second rays of the image light coupled into thewaveguide at the second propagation angle propagate toward the outputarea within the middle portion of the waveguide body.

Referring to FIG. 16, in some embodiments a multi-layer orgradient-index waveguide, such as a two-layer waveguide 1102 illustratedby way of example, may be combined with a second waveguide 1101 to forma waveguide stack 1100. Waveguide 1102 may be as described above withreference to any of FIGS. 9A-15. The second waveguide may be asingle-layer waveguide of the type shown in FIGS. 2-4, or any of themulti-layer or gradient-index waveguide described above with therefractive index varying in the direction of the waveguide thickness. Agap may separate the two-layer waveguide 1102 and the second waveguide1101. The gap may be an airgap, a vacuum gap, or it may include amaterial of lower refractive index than that of the waveguides. Thein-couplers 1131, 1132 and out-couplers 1141, 1142 of the waveguides maybe optimized for different wavelength ranges. For example, thein-coupler 1132 and the out-coupler 1142 of the two-layer waveguide 1102may be optimized to couple light of the blue and green color channels,while the in-coupler 1131 and the out-coupler 1141 of the secondwaveguide 1101 may be optimized to couple light of the red colorchannel. In some embodiments light of one or more color channels may beconveyed by each of the two waveguides, for example depending on theangle of incidence.

FIG. 17A schematically illustrates an example layout of a binocularnear-eye display (NED) 1200 that includes two waveguide assemblies 1210supported by a frame or frames 1215. Each of the waveguide assemblies1210 is configured to convey image light from a display projector 1260to a different eye of a user. The in-couplers 1230 may be provided witha common micro-display projector or two separate micro-displayprojectors 1260, which may be disposed to project image light toward thecorresponding in-couplers 1230. Waveguide assemblies 1210 may each be inthe form of, or include, a single multi-layer or gradient-indexwaveguide that may be configured to guide polychromatic light in atarget FOV as described above. Each waveguide includes an in-coupler1230 and an out-coupler 1240, with each in-coupler diagonally alignedwith the corresponding out-coupler. In other embodiments the placementof the in-couplers 1230 in the periphery of the correspondingout-couplers 1240 may be different. Each out-coupler 1240 includes aneyebox projection area 1251, which may also be referred to as the exitpupil of the waveguide, and from which in operation the image light isprojected to an eye of the user. An eye box is a geometrical area wherea good-quality image may be presented to a user's eye, and where inoperation the user's eye is expected to be located. The eyeboxprojection areas 1251 may be disposed on an axis 1201 that connectstheir centers. The axis 1201 may be suitably aligned with the eyes ofthe user wearing the NED, or be at least parallel to a line connectingthe eyes of the user, and may be referred to as the horizontal axis(x-axis). The in-couplers 1230 may be in the form of diffractiongratings with grating vectors go that may be directed generally towardthe eyebox projection areas 1251 of respective waveguide assemblies.Each out-coupler 1240 may be in the form of two diffraction gratings,with the grating vectors g₁ and g₂ of the respective gratings orientedat an angle to each other. These gratings may be disposed at high-indexportions of each waveguide to couple image light therein, and tode-couple light propagating in the high-index layer or region out of thewaveguide as described above. The gratings of the in-coupler andout-coupler may be configured to satisfy a vector diagram illustrated inFIG. 17B. In some embodiments each waveguide assembly 1210 may be in theform of, or include, a waveguide stack with two or more waveguides asdescribed above, with the grating vectors g₀, g₁ and g₂ that may bedifferent in length for each waveguide of the stack and may be optimizedfor conveying different color channels. In some embodiments the gratingsof each waveguide of the stack may be configured so at to avoid, or atleast lessen, the leakage of once-diffracted ambient light into thesupported FOV, or at least a pre-defined central portion of thesupported FOV, as described above.

Embodiments of the present disclosure may include, or be implemented inconjunction with, an artificial reality system. An artificial realitysystem adjusts sensory information about outside world obtained throughthe senses such as visual information, audio, touch (somatosensation)information, acceleration, balance, etc., in some manner beforepresentation to a user. By way of non-limiting examples, artificialreality may include virtual reality (VR), augmented reality (AR), mixedreality (MR), hybrid reality, or some combination and/or derivativesthereof. Artificial reality content may include entirely generatedcontent or generated content combined with captured (e.g., real-world)content. The artificial reality content may include video, audio,somatic or haptic feedback, or some combination thereof. Any of thiscontent may be presented in a single channel or in multiple channels,such as in a stereo video that produces a three-dimensional effect tothe viewer. Furthermore, in some embodiments, artificial reality mayalso be associated with applications, products, accessories, services,or some combination thereof, that are used to, for example, createcontent in artificial reality and/or are otherwise used in (e.g.,perform activities in) artificial reality. The artificial reality systemthat provides the artificial reality content may be implemented onvarious platforms, including a wearable display such as an HMD connectedto a host computer system, a standalone HMD, a near-eye display having aform factor of eyeglasses, a mobile device or computing system, or anyother hardware platform capable of providing artificial reality contentto one or more viewers.

Referring to FIG. 18A, an HMD 1300 is an example of an AR/VR wearabledisplay system which encloses the user's face, for a greater degree ofimmersion into the AR/VR environment. The HMD 1300 may be an embodimentof the waveguide display 100 of FIG. 1A or the NED 1200 of FIG. 17A, forexample. The function of the HMD 1300 is to augment views of a physical,real-world environment with computer-generated imagery, and/or togenerate the entirely virtual 3D imagery. The HMD 1300 may include afront body 1302 and a band 1304. The front body 1302 is configured forplacement in front of eyes of a user in a reliable and comfortablemanner, and the band 1304 may be stretched to secure the front body 1302on the user's head. A display system 1380 may be disposed in the frontbody 1302 for presenting AR/VR imagery to the user. Sides 1306 of thefront body 1302 may be opaque or transparent. The display system 1380may include a display waveguide as described above coupled to imageprojectors 1314.

In some embodiments, the front body 1302 includes locators 1308 and aninertial measurement unit (IMU) 1310 for tracking acceleration of theHMD 1300, and position sensors 1312 for tracking position of the HMD1300. The IMU 1310 is an electronic device that generates dataindicating a position of the HMD 1300 based on measurement signalsreceived from one or more of position sensors 1312, which generate oneor more measurement signals in response to motion of the HMD 1300.Examples of position sensors 1312 include: one or more accelerometers,one or more gyroscopes, one or more magnetometers, another suitable typeof sensor that detects motion, a type of sensor used for errorcorrection of the IMU 1310, or some combination thereof. The positionsensors 1312 may be located external to the IMU 1310, internal to theIMU 1310, or some combination thereof.

The locators 1308 are traced by an external imaging device of a virtualreality system, such that the virtual reality system can track thelocation and orientation of the HMD 1300. Information generated by theIMU 1310 and the position sensors 1312 may be compared with the positionand orientation obtained by tracking the locators 1308, for improvedtracking accuracy of position and orientation of the HMD 1300. Accurateposition and orientation is important for presenting appropriate virtualscenery to the user as the latter moves and turns in 3D space.

The HMD 1300 may further include a depth camera assembly (DCA) 1311,which captures data describing depth information of a local areasurrounding some or all of the HMD 1300. To that end, the DCA 1311 mayinclude a laser radar (LIDAR), or a similar device. The depthinformation may be compared with the information from the IMU 1310, forbetter accuracy of determination of position and orientation of the HMD1300 in 3D space.

The HMD 1300 may further include an eye tracking system for determiningorientation and position of user's eyes in real time. The determinedposition of the user's eyes allows the HMD 1300 to perform (self-)adjustment procedures. The obtained position and orientation of the eyesalso allows the HMD 1300 to determine the gaze direction of the user andto adjust the image generated by the display system 1380 accordingly. Inone embodiment, the vergence, that is, the convergence angle of theuser's eyes gaze, is determined. The determined gaze direction andvergence angle may also be used for real-time compensation of visualartifacts dependent on the angle of view and eye position. Furthermore,the determined vergence and gaze angles may be used for interaction withthe user, highlighting objects, bringing objects to the foreground,creating additional objects or pointers, etc. An audio system may alsobe provided including e.g. a set of small speakers built into the frontbody 1302.

Referring to FIG. 18B, an AR/VR system 1350 may be an exampleimplementation of the waveguide display 100 of FIG. 1A, or the NED 1200of FIG. 17A. The AR/VR system 1350 includes the HMD 1300 of FIG. 18A, anexternal console 1390 storing various AR/VR applications, setup andcalibration procedures, 3D videos, etc., and an input/output (I/O)interface 1315 for operating the console 1390 and/or interacting withthe AR/VR environment. The HMD 1300 may be “tethered” to the console1390 with a physical cable, or connected to the console 1390 via awireless communication link such as Bluetooth®, Wi-Fi, etc. There may bemultiple HMDs 1300, each having an associated I/O interface 1315, witheach HMD 1300 and I/O interface(s) 1315 communicating with the console1390. In alternative configurations, different and/or additionalcomponents may be included in the AR/VR system 1350. Additionally,functionality described in conjunction with one or more of thecomponents shown in FIGS. 18A and 18B may be distributed among thecomponents in a different manner than described in conjunction withFIGS. 18A and 18B in some embodiments. For example, some or all of thefunctionality of the console 1315 may be provided by the HMD 1300, andvice versa. The HMD 1300 may be provided with a processing modulecapable of achieving such functionality.

As described above with reference to FIG. 18A, the HMD 1300 may includean eye tracking system 1325 for tracking eye position and orientation,determining gaze angle and convergence angle, etc., the IMU 1310 fordetermining position and orientation of the HMD 1300 in 3D space, theDCA 1311 for capturing the outside environment, the position sensor 1312for independently determining the position of the HMD 1300, and thedisplay system 1380 for displaying AR/VR content to the user. Thedisplay system 1380 includes (FIG. 18B) one or more image projectors1314, such as one or more scanning projectors or one or more electronicdisplays, including but not limited to a liquid crystal display (LCD),an organic light emitting display (OLED), an inorganic light emittingdisplay (ILED), an active-matrix organic light-emitting diode (AMOLED)display, a transparent organic light emitting diode (TOLED) display, aprojector, or a combination thereof. The display system 1380 furtherincludes a display waveguide 1330, whose function is to convey theimages generated by the image projector 1314 to the user's eye. Thedisplay system 1380 may further include an optics block 1335, which mayin turn include various lenses, e.g. a refractive lens, a Fresnel lens,a diffractive lens, an active or passive Pancharatnam-Berry phase (PBP)lens, a liquid lens, a liquid crystal lens, etc., a pupil-replicatingwaveguide, grating structures, coatings, etc. In some embodiments theoptics block 1335 may include a varifocal functionality a varifocalmodule to adjust the focus of the optics block 1330 e.g. to compensatefor vergence-accommodation conflict, to correct for vision defects of aparticular user, to offset aberrations, etc.

The I/O interface 1315 is a device that allows a user to send actionrequests and receive responses from the console 1390. An action requestis a request to perform a particular action. For example, an actionrequest may be an instruction to start or end capture of image or videodata or an instruction to perform a particular action within anapplication. The I/O interface 1315 may include one or more inputdevices, such as a keyboard, a mouse, a game controller, or any othersuitable device for receiving action requests and communicating theaction requests to the console 1390. An action request received by theI/O interface 1315 is communicated to the console 1390, which performsan action corresponding to the action request. In some embodiments, theI/O interface 1315 includes an IMU that captures calibration dataindicating an estimated position of the I/O interface 1315 relative toan initial position of the I/O interface 1315. In some embodiments, theI/O interface 1315 may provide haptic feedback to the user in accordancewith instructions received from the console 1390. For example, hapticfeedback can be provided when an action request is received, or theconsole 1390 communicates instructions to the I/O interface 1315 causingthe I/O interface 1315 to generate haptic feedback when the console 1390performs an action.

The console 1390 may provide content to the HMD 1300 for processing inaccordance with information received from one or more of: the IMU 1310,the DCA 1311, the eye tracking system 1325, and the I/O interface 1315.In the example shown in FIG. 18B, the console 1390 includes anapplication store 1355, a tracking module 1360, and a processing module1365. Some embodiments of the console 1390 may have different modules orcomponents than those described in conjunction with FIG. 18B. Similarly,the functions further described below may be distributed amongcomponents of the console 1390 in a different manner than described inconjunction with FIGS. 18A and 18B.

The application store 1355 may store one or more applications forexecution by the console 1390. An application is a group of instructionsthat, when executed by a processor, generates content for presentationto the user. Content generated by an application may be in response toinputs received from the user via movement of the HMD 1300 or the I/Ointerface 1315. Examples of applications include: gaming applications,presentation and conferencing applications, video playback applications,or other suitable applications.

The tracking module 1360 may calibrate the AR/VR system 1350 using oneor more calibration parameters and may adjust one or more calibrationparameters to reduce error in determination of the position of the HMD1300 or the I/O interface 1315. Calibration performed by the trackingmodule 1360 also accounts for information received from the IMU 1310 inthe HMD 1300 and/or an IMU included in the I/O interface 1315, if any.Additionally, if tracking of the HMD 1300 is lost, the tracking module1360 may re-calibrate some or all of the AR/VR system 1350.

The tracking module 1360 may track movements of the HMD 1300 or of theI/O interface 1315, the IMU 1310, or some combination thereof. Forexample, the tracking module 1360 may determine a position of areference point of the HMD 1300 in a mapping of a local area based oninformation from the HMD 1300. The tracking module 1360 may alsodetermine positions of the reference point of the HMD 1300 or areference point of the I/O interface 1315 using data indicating aposition of the HMD 1300 from the IMU 1310 or using data indicating aposition of the I/O interface 1315 from an IMU included in the I/Ointerface 1315, respectively. Furthermore, in some embodiments, thetracking module 1360 may use portions of data indicating a position orthe HMD 1300 from the IMU 1310 as well as representations of the localarea from the DCA 1311 to predict a future location of the HMD 1300. Thetracking module 1360 provides the estimated or predicted future positionof the HMD 1300 or the I/O interface 1315 to the processing module 1365.

The processing module 1365 may generate a 3D mapping of the areasurrounding some or all of the HMD 1300 (“local area”) based oninformation received from the HMD 1300. In some embodiments, theprocessing module 1365 determines depth information for the 3D mappingof the local area based on information received from the DCA 1311 thatis relevant for techniques used in computing depth. In variousembodiments, the processing module 1365 may use the depth information toupdate a model of the local area and generate content based in part onthe updated model.

The processing module 1365 executes applications within the AR/VR system1350 and receives position information, acceleration information,velocitynformation, predicted future positions, or some combinationthereof, of the HMD 1300 from the tracking module 1360. Based on thereceived information, the processing module 1365 determines content toprovide to the HMD 1300 for presentation to the user. For example, ifthe received information indicates that the user has looked to the left,the processing module 1365 generates content for the HMD 1300 thatmirrors the user's movement in a virtual environment or in anenvironment augmenting the local area with additional content.Additionally, the processing module 1365 performs an action within anapplication executing on the console 1390 in response to an actionrequest received from the I/O interface 1315 and provides feedback tothe user that the action was performed. The provided feedback may bevisual or audible feedback via the HMD 1300 or haptic feedback via theI/O interface 1315.

In some embodiments, based on the eye tracking information (e.g.,orientation of the user's eyes) received from the eye tracking system1325, the processing module 1365 determines resolution of the contentprovided to the HMD 1300 for presentation to the user with the imageprojector(s) 1314. The processing module 1365 may provide the content tothe HMD 1300 having a maximum pixel resolution in a foveal region of theuser's gaze. The processing module 1365 may provide a lower pixelresolution in the periphery of the user's gaze, thus lessening powerconsumption of the AR/VR system 1350 and saving computing resources ofthe console 1390 without compromising a visual experience of the user.In some embodiments, the processing module 1365 can further use the eyetracking information to adjust where objects are displayed for theuser's eye to prevent vergence-accommodation conflict and/or to offsetoptical distortions and aberrations.

The hardware used to implement the various illustrative logics, logicalblocks, modules, and circuits described in connection with the aspectsdisclosed herein may be implemented or performed with a general purposeprocessor, a digital signal processor (DSP), an application specificintegrated circuit (ASIC), a field programmable gate array (FPGA) orother programmable logic device, discrete gate or transistor logic,discrete hardware components, or any combination thereof designed toperform the functions described herein. A general-purpose processor maybe a microprocessor, but, in the alternative, the processor may be anyconventional processor, controller, microcontroller, or state machine. Aprocessor may also be implemented as a combination of computing devices,e.g., a combination of a DSP and a microprocessor, a plurality ofmicroprocessors, one or more microprocessors in conjunction with a DSPcore, or any other such configuration. Alternatively, some steps ormethods may be performed by circuitry that is specific to a givenfunction.

The present disclosure is not to be limited in scope by the specificembodiments described herein. Indeed, other various embodiments andmodifications, in addition to those described herein, will be apparentto those of ordinary skill in the art from the foregoing description andaccompanying drawings. Thus, such other embodiments and modificationsare intended to fall within the scope of the present disclosure.Further, although the present disclosure has been described herein inthe context of a particular implementation in a particular environmentfor a particular purpose, those of ordinary skill in the art willrecognize that its usefulness is not limited thereto and that thepresent disclosure may be beneficially implemented in any number ofenvironments for any number of purposes. Accordingly, the claims setforth below should be construed in view of the full breadth and spiritof the present disclosure as described herein.

What is claimed is:
 1. A waveguide for conveying image light in adisplay system, the waveguide comprising: a waveguide body having twoouter surfaces and a thickness therebetween, the waveguide bodycomprising an input area and an output area, the waveguide bodyconfigured to guide the image light received at the input area towardthe output area, wherein the waveguide body has a refractive index thatvaries in a direction of the thickness and is greater in a middleportion of the waveguide body between the outer surfaces than inportions of the waveguide body adjacent to the outer surfaces; an inputcoupler disposed in the input area and configured to couple the imagelight into the waveguide body for propagating toward the output area;and, an output coupler disposed in the output area and configured tocouple the image light out of the waveguide body for propagating towardan eyebox.
 2. The waveguide of claim 1 wherein the input coupler isconfigured to couple the image light into the waveguide body in a rangeof propagation angles comprising a first propagation angle and a secondpropagation angle, so that first rays of the image light coupled intothe waveguide body at the first propagation angle are guided toward theoutput area by total internal refraction at the outer surfaces of thewaveguide body, while second rays of the image light coupled at thesecond propagation angle are guided within the middle portion of thewaveguide body.
 3. The waveguide of claim 1 wherein the input couplercomprises a first input grating disposed at least in part in the middleportion of the waveguide body so as to inject the image light directlyinto the middle portion.
 4. The waveguide of claim 1 wherein the outputcoupler comprises a first output grating disposed at least in part inthe middle portion of the waveguide body.
 5. The waveguide of claim 4wherein the output coupler comprises a second output grating configuredto redirect the image light received from the first output grating outof the waveguide body.
 6. The waveguide of claim 5 wherein the secondoutput grating is disposed at least in part in the middle portion of thewaveguide body.
 7. The waveguide of claim 5 wherein the middle portionof the waveguide body comprises a middle layer, the waveguiding bodyfurther comprising two outer layers sandwiching the middle layer,wherein the middle layer has a greater refractive index than the twoouter layers.
 8. The waveguide of claim 5 wherein the waveguide bodycomprises a plurality of layers, the plurality of layers comprising atleast three layers having different refractive indices.
 9. The waveguideof claim 7 wherein at least one of the first and second output gratingsis disposed in the middle layer or at a surface thereof.
 10. Thewaveguide of claim 7 wherein the first input grating is disposed in themiddle layer or at a surface thereof.
 11. The waveguide of claim 1wherein the waveguide body is contiguous.
 12. The waveguide of claim 1wherein the refractive index of the waveguide body varies from at least2.3 in the middle portion to at most 1.9 at the outer surfaces thereof.13. The waveguide of claim 1 wherein the refractive index of thewaveguide body gradually varies in the thickness dimension, decreasingfrom a maximum value in the middle portion of the waveguide body towardlesser values at the two outer surfaces thereof.
 14. A near-eye display(NED) device comprising: a support structure for wearing on a head of auser; a light projector carried by the support structure and configuredto emit image light; and, a first waveguide carried by the supportstructure and configured to convey at least a first portion of the imagelight from the light projector to an eyebox, the first waveguidecomprising: a waveguide body having two outer surfaces and a thicknesstherebetween, the waveguide body comprising an input area and an outputarea, the waveguide body configured to guide the image light received atthe input area toward the output area, wherein the waveguide body has arefractive index that varies in a direction of the thickness and isgreater in a middle portion of the waveguide body between the outersurfaces than in portions of the waveguide body adjacent to the outersurfaces; an input coupler configured to couple the first portion of theimage light into the waveguide body; and, an output coupler configuredto couple the first portion of the image light out of the waveguide bodyfor propagating toward an eyebox.
 15. The NED device of claim 14,wherein the input coupler comprises a first input grating disposed atleast in part in the middle portion of the waveguide body, and theoutput coupler comprises a first output grating disposed at least inpart in the middle portion of the waveguide body, wherein the firstoutput grating is spaced apart from the first input grating in a firstdirection that is orthogonal to a direction of the thickness.
 16. TheNED device of claim 14 wherein the image light comprises a plurality ofcolor channels, and wherein the first waveguide is configured to coupleat least a first color channel of the image light for guiding toward theeyebox.
 17. The NED device of claim 14 comprising a second waveguidecarried by the support structure and configured to convey at least asecond portion of the image light from the light projector to theeyebox, wherein the image light comprises a first color channel and asecond color channel, and wherein the first waveguide is configured totransmit the first color channel and the second waveguide is configuredto transmit the second color channel.
 18. A method for conveying imagelight from an image light source to an eyebox, the method comprising:directing the image light onto an input area of a waveguide comprisingtwo opposed outer surfaces and a middle portion therebetween, the middleportion extending from the input area of the waveguide to an output areathereof, the middle portion having a greater refractive index thanportions of the waveguide body adjacent to the opposed outer surfacesthereof; coupling the image light into the waveguide body at a range ofpropagation angles comprising a first propagation angle and a secondpropagation angle; and, propagating the image light in the waveguidefrom the input area to the output area so that first rays of the imagelight coupled into the waveguide at the first propagation angle areguided toward the output area by total internal refraction at theopposed outer surfaces of the waveguide, while second rays of the imagelight coupled into the waveguide at the second propagation anglepropagate toward the output area within the middle portion of thewaveguide body.
 19. The method of claim 18 comprising using at least oneoutput grating disposed in the middle portion of the waveguide body tocouple the second rays out of the waveguide body.
 20. The method ofclaim 18 comprising guiding at least one color channel of the imagelight predominantly within the middle portion of the waveguide.